Electrical resistor and a method of making the same



1954 F. E. KERRIDGE ETAL ,6

ELECTRICAL RESISTOR A METHOD OF MAKING THE SAME Filed June 19, 1951 2 Sheets-Sheet l Hal.

- Int/91150115 501m Evocw fl/Ffi/DGE GRAHAM P541965:

*4 Attorneys 1954 F; E. KERRIDGE ETAL ,693,023

ELECTRICAL RESISTOR AND A METHOD OF MAKING THE SAME Filed June 19, 1951 2 Sheets-Sheet 2 FIG 10 g g ggggggggg I nuit "Ill United States Patent ELEQTRHIAL RESISTOR AND A METHOD OF MAKENG THE SAh IE Frank Kerridge, London, and Graham Pearce, Kingsthorpc, Northamptcn, England, assignors, by mesne assignments, to Painton & Company, Limited, Kingsthorpe, Northampton, England, a British com- P y Application June 19, 1951, Serial No. 232,382 Claims priority, application Great Britain June 20, 1950 E (Claims. (Cl. 29-4555) This invention relates to improvements in the art of manufacturing electrical resistors.

The principal object of the invention is to provide a novel method of making a flat-type metal film resistor.

A further object of the invention is to provide a method of making an electrical resistor which shall be of high value and stable in use.

Another object of the invention is to enable an electrical resistor to be provided which ensures complete metallic conduction throughout the whole range of its effective resistance value.

Another object is to provide an electrical resistor having a high resistance value of the order of 10 to ohms.

Yet another object of the invention is to provide a resistor which shall be stable and at the same time have a high resistance value.

A further object is to provide an electrical resistor having a resistance element in the form of a thin metallic film which is substantially uniformly distributed over a non-metallic hacking sheet.

Another object is to enable a thin stable metallic film to be given a high resistance value by tracking.

A still further object of the invention is to provide a high value electrical resistor in which the current path consists of a meandering metal ribbon.

Another and important object of the invention is to provide a method of manufacture of flat metal film electrical resistors which enables such resistors to be made rapidly and economically by mass production methods, the resistance value of the resistors being accurately reproducible or varied as desired from batch to batch.

The manner in which the above and other objects are attained will be clearly understandable from the following description taken in conjunction with the accompanying drawings, which show the preferred manner of carrying out the invention, it being clearly understood that the invention is in no way limited thereto or thereby, but that the true scope of the invention is defined in and by the appended claims.

The phenomenal increase in electronic devices in the military, industrial and social spheres has, during the past decade, stimulated intense research for an electrical resistor having a. high degree of stability and which, at the same time, is capable of being produced economically in large quantities.

It is fundamental that the resistance value of a high stability resistor must remain substantially the same under diverse operating conditions. The resistance must not vary more than 21% and in some cases even i0.2% of its original value when under electrical load for several months, or when subjected to repeated cycles of combined high humidity and high temperature followed by low temperature. The standard requirements for such resistors are laid down in Service Specifications J an. R-93 for Grade 1 resistors. In addition to high stability, such resistors are required to have low positive temperature coefiicients and should be substantially free from noise in operation. Furthermore, owing to the modern tendency to reduce the size of components in electrical circuits, such as the recently developed printed circuits, it is very desirable, if not essential, that resistors shall taining carbon and palladium oxide. to produce high value resistors with be small in size and of such a shape as to fit conveniently into any type of circuit.

High stability resistors have hitherto been produced by making each resistor as an individual unit, and the processes involved have, in most cases, required a high degree of manual skill. For instance, high stability nonreactive resistors of up to 100,000 ohms resistance have been available for a number of years and these are produced by winding specially prepared alloy wires on to individual formers, usually of circular cross-section. The properties of the wire are permanent and reproducible and the wire provides what is known as metallic conduction, which imparts high stability to the resistor.

in the manufacture of these resistors, however, very fine wires are required and much time and care are needed in winding the wire on to the individual formers.

Resistors of higher value, say from 100,000 ohms to 5 megohms, have not hitherto been available With the same inherent stability as the aforementioned wire wound resistor. Such high value resistors as are marketed usually employ thin films containing carbon, or palladium oxide for the conducting path. These films do not, however, provide metallic conduction, but involve the condition known as semi-conduction and their stability is adversely affected by gases or moisture.

in a metallic conductor the metal or alloy has a uniform and fixed structure throughout the conducting path and electrons can pass regularly through the lattice. In a semi-conductor, traces of impurity, including occluded gases, or slight rearrangement of the structure alter the rbalte of conduction, thus detrimentally afiecting the sta- 1 ity.

Resistors embodying such films, therefore, have to be completely sealed in glass envelopes in order to prevent the ingress of gases and moisture and to provide high stability. This procedure is diflicult and costly and again necessitates individual treatment of each resistor.

Our invention, accordingly, concerns the manufacture of very high stability resistors employing metallic conduction throughout the whole range of resistance values and provides for the first time a method for the economical production of resistors in large quantities. Broadly our invention consists in constituting the conducting path as a thin adherent film of a stable metal or alloy uniformly distributed over the surface of a substantially fiat insulating base and by increasing the resistance of the film, preferably by a photo-etching process, so as to narrow the current path.

Conventional resistors are cylindrical in shape, but this shape is inherent in the method of manufacture employed. Our method produces fiat resistors having dimensions of 1" X /2 or less and although this shape has so far not found use commercially, it has many advantages. For example, a number of such resistors may be stacked in a small space, allowing high resistance values to be obtained with economy of space, and their fiat contour allows them to be fitted readily into printed circuits.

For our alloy film conducting path, we use a gold: platinum alloy film. Gold and platinum are extremely stable chemically and electrically and are not subject to oxidation at room temperatures as are most base metals, or to oxidation at elevated temperatures as are some of the other noble metals. Goldzplatinum alloys in the proportions of -90 parts of gold to 1040 parts of platinum have been found particularly suitable for our purpose and from this range of alloys we prefer an :20 goldzplatimun alloy since it has the lowest positive temperature coeflicient in conjunction with high stability. Films formed with such an alloy have, however, a very low resistivity compared with films con- Therefore, in order this alloy we were faced with the problem of attenuating the current path into a fine ribbon of from .001-.005" in width so as to provide high resistance values. A further problem was ice 7 to ensure that an even film was deposited on the insulating base to ensure reproducibility of values and to avoid local overheating resulting in change in resistance value or breakdown of the resistors in use.

Deposition of a thin even film and attenuation of the current path had also to be carried out in a manner suitable for large scale production in accordance with one of the important objects of our invention. These problems were solved in the fol-lowing manner:

Even deposition of the metallic film was effected by using a solution containing the sulphoresinates of the two metals and applying this to a substantially flat sheet of a non-conducting heat-resisting material by the spinning method, i. e. applying a quantity to the surface of the sheet and rotating the latter at a high speed in a horizontal plane around a vertical axis. The evenly coated sheet is subsequently fired to produce a very thin even metallic film.

Attenuation of the film by mechanical tracking, although satisfactory for small quantities, did not lend itself to quantity production since it had to be carried out on individual resistors and one of our important aims in this work was to endeavour to produce a plurality of resistors in one operation. A photoetching process in which the required attenuated pattern is produced photographically on the metallised surface in the form of a resist through which it is possible to etch the unwanted metal image-wise seemed more promising, and complete success was eventually achieved with this method. It was found possible to provide by this method from 20-100 attenuated resistor current paths with good reproducibility on a single sheet and at the end of the photoetching stage the resistance elements had values of the desired final value. The problem of final adjustment to i15% of desired value was solved by providing in the photo-etch pattern a number of loops connected in parallel to one terminal of the resistor and cutting through one or more of the parallel connections by placing the resistor element in an adjusting jig and moving a scriber to cut through the requisite number of loops in the photoetch pattern, at the same time measuring the resistance value of the element and stopping the scribing operation when the desired value is reached.

The above series of operations enables highly stable resistors up to 10 megohms .to 'be obtained and is em1-v nently suitable for quantity production, since a plurality of resistors can be produced on a single sheet and subsequently cut into individual units. Suitable terminal clips and connecting leads are attached by clipping and soldering to the end of each unit and the surface is protected with a moisture-proof insulating varnish to complete the resistor. Since the current path of the resistor is a metal alloy with metallic conduction, a very high degree of electrical stability is obtained which is comparable to that obtained with the wire-wound type of resistor.

From a consideration of the above discussion of the problems that confronted us and the detailed manner in which we eventually overcame them, certain particular features emerge, which will now, for convenience, be shortly stunmarised.

Firstly, in order to ensure that our resistor shall exhibit as high a degree of electrical stability as possible, we employ as our metallising film agold1platinum alloy. These alloys are in general electrically stable and at the same time provide the desirable metallic conduction above referred to. We have found that the best results are obtained by the use of an 80:20 goldzplatinum alloy, which has a low, positive temperature coefiicient of resistance.

In the application of the goldcplatinurn alloy metallising film to the insulated base, the film should be applied in the form of a solution comprising a mixture of a compound of gold and a compound of platinum in the proportions of 60-90 parts of gold to 10-40 parts of platinum dissolved in a suitable solvent mixture and the coated sheet subsequently fired to burn away. the organic matter and cause the resultant alloy film to adhere firmly to the sheet. In practice we have found that a mixture of the sulphoresinates of gold and platinum in1 the required proportions gives very satisfactory resu ts. 1 1

As it is essential for the purposes of our invention that the metallising film shall be distributed as evenly as possible over the surface of the base, we prefer to apply the film by a whirling or spinning method. By this means a thin film of the solution becomes evenly distributed over the whole surface area.

So far we have, by the above manner of procedure,

obtained .a .thin uniform ,film of about .one millionth of an inch thickness of an alloy which gives complete metallic conduction and a very high degree of stability. The alloy film, however, has a very low resistivity so that, in order to produce a high value in accordance with one of the main purposes of the present invention, it becomes necessary to attenuate the current path by tracking the alloy film. This we effect by a photoetching method, as we have found that, by this method, it becomes .possible to obtain an extremely fine ribbonlike conducting path of about 0.00l-0.005 inch in width. Extremely high resistance values can then be obtained as a conducting track of at least six to eight feet long can be provided on a resistor having a surface area of l" V2".

The insulating base or backing sheet to which the alloy film is applied may be formed of any suitable nonconducting, heat-resisting material such as glass, ceramic or mica. We have found, however, that the most satisfactory results are obtained by the use of glass. This is particularly the case when our improved method is employed for the mass production of electrical resistors as hereinafter fully described, as it renders particularly simple the cutting of the metallised and treated sheet into a plurality of individual resistor elements.

In order to facilitate the attachment of conducting leads, it will be found desirable, although not essential, to provide thickening strips or bands of conductive material along opposite sides of the resistor corresponding to the opposed ends of the tracked current path. This may advantageously be effected by applying to the tracked sheet adjacent opposite edges thereof, a band of a metallising paste e. g. silver paste, which is then fired in the usual manner to produce adhesion of the silver and burning away of the organic matter in the paste.

When thickening bands, as above referred to, are to be applied, .the final firing step may advantageously be deferred until after the application of the said bands, and a single firing step employed.

Means is provided for enabling final adjustment of the resistance value of the .current path to be efiiected. For this purpose, the tracked current path incorporates at one end thereof a plurality of loops connected in parallel to one terminal of the resistor circuit, whereby additional resistance may be included in the circuit by cutting through, for example, by means of a mechanical scribing device, one or more of the parallel connections as required.

As pointed out above, our improved method is part cularly suitable for the mass production of electrical resistors-and, accordingly, a still more limited aspect of the invention comprises the steps 'of coacting a substantially flat sheet of non-conducting heat-resisting material such as glass on at least one surface thereof with an adherent uniform thin film of a gold z-platinum alloy in the proportions of 6090 parts of gold to 10-40 parts of platinum; tracking saidalloy film by a photoetching process to provlde a plurality of individual spaced narrow ribbon-like current paths, each including a plurality of parallelly arranged looped sections; applying to said sotreated sheet bands of metal separating said individual track designs; dividing the sheet into elongated strips by cutting substantially along the centre line between each of sa1d ad acent metal bands; then cutting each strip substantially transversely of the length thereof to form a plurahty of individual tracked resistor-s having two opposed thickened edges forming terminals and finally ad usting the resistance value of each resistor as requ1red by introducing one or more of said looped sectrons mto the current path.

In carrying out our invention, therefore, according to the preferred mode of procedure thereof, We first provide a circular glass disc cut to a uniform predetermined size; and, after ensuring that the surface to which the metall sing film is to be applied is chemically clean by immersmg t in acid, we then apply .to the .clean surface, WhlCh is held in .a horizontal position, a solution comprising a mixture of the sulphoresinate of gold and the sulphoresmate of platinum dissolved in an essential oil solvent mixture and rotate the glass disc at a high speed around .a vertical axis.

The coated disc is then fired at a suitable temperature to burn away the organic matter and cause the resulting goldzplatinum alloy film to adhere to the glass disc.

The desired track design is then formed in the alloy film by a photoetching process, thickening strips of metal, e. g. silver, are applied, if desired, to the opposed edges of the track design, external lead wires are soldered to the thickened portions or to the opposed ends of the tracked conducting path, it thickened portions are not provided, and the resistance value of the resistor is finally adusted by scribing in the manner referred to above.

The completed resistor is then given one or more coatings of a moisture-proof insulating varnish, such as a silicone varnish.

The following is an example of a suitable gold: platinum solution for use in forming the metallising fihn:

Parts GOld sulphoresinate 280 Platinum sulphoresinate 140 Rhodium sulphoresinate 60 Vanadium resinate 70 Boric acid solution in benzyl alcohol 10 Cyclohexanol 250 Colophony 100 Oil of lavender 20 Oil of rosemary 20 Spirits of turpentine 20 Nitrobenzol 30 This solution has the following percentage composition, namely:

Solvents and resinous constituents 92.65

In the use of the above solution for the quantity production of electrical resistors we proceed in the following manner.

(1) Coating or metallising step A 4 1;" diameter circular disc of soda lime glass is immersed in a 20% aqueous solution of chromic acid for 48 hours, washed in distilled water, and allowed to dry. This procedure ensures that the glass is chemically clean before receiving the metallising solution.

The cleaned glass disc is then clamped in position on a spinning table which is enclosed in a cabinet, the interior of which is maintained at a temperature of 23 C. The spinning table is geared to an electric motor which enables it to be rotated in a horizontal plane around a vertical axis. 4 ccs. of the above metallising solution are placed in the centre of the glass disc, which is then rotated at a speed of 1800 revolutions per minute for minutes. At the end of this period the metallising solution will have spread as a thin even film over the surface of the disc and the rapid whirling at relatively high temperature will have dried the film so that the disc may be taken from the cabinet without damage to the film.

(2) Firing step The coated disc is now placed in a furnace and fired to a temperature of 420 C., allowing 70 minutes from room temperature to peak and 20 minutes at peak temperature. After cooling, the resistance of the fired-on alloy film will be found to be about 20 ohms per square.

(3) Photoetching step 4 ccs. of dichromate-sensitised photoengraving glue are applied to the metallised glass disc, which is again rotated at 1800 R. P. M. for 5 minutes at a temperature of 23 C. This imparts a thin even film of glue to the metallised surface of the disc.

A photographic negative of the desired multiple resistor track designs including looped sections is then placed in contact with the light sensitive glue film, which is then exposed to light through the said negative for a period of between 2 and minutes, depending on the type of light source employed and the distance of the glass disc from the said source.

The exposed disc is then developed by washing for 3 minutes in cold water and allowed to dry. The disc is then stoved at a temperature of 280300 C. for

20 minutes to harden the glue image and convert it into a resist.

The disc is then placed in a buttered halogen etching bath composed of 98% amyl lactate and 2% bromine for about 10 minutes at a temperature of 30 C. until the alloy film is removed from the portions of the disc not protected by the glue image.

After the etching operation has been completed, the disc is rinsed in water and allowed to dry.

(4) Applying contacts Opposite bands of silver paste are applied between the rows of track designs by the screen printing process. The disc is then refired at a temperature of 600 C. to burn away the glue resist and to cause the silver paste to form an adherent film of silver. The retired disc is then cut to form a plurality of individual tracked resistors and end caps are applied and soldered in position.

(5) Adjustment of resistance value The resistors are finally adjusted to the required value by scribing through one or more of the aforesaid looped sections in the resistor design and the completed resistors are then finally given one or more coatings of insulating moisture-proof silicone resin varnish.

In the accompanying drawings:

Figure l is a sectional end view of a glass sheet from which the resistor is to be made,

Figure 2 is a similar view showing a thin film of an 80:20 gold:platinurn alloy applied to the upper surface of the glass sheet,

Figure 3 shows the metallised glass sheet with a layer of light-sensitised glue applied to the alloy coating preparatory to photoetching the desired track design thereon,

Figure 4 shows the article after an image of the track design has been photographically produced in the glue layer and the unaffected glue has been removed,

Figure 5 shows the next stage in the method after the alloy film not covered by the image has been etched away,

Figure 6 shows the track design after firing to burn away the glue resist,

Figure 7 is a plan view of the resistor showing the arrangement of the tracked design, and including terminal clips,

Figure 8 illustrates a further step in the process of making the finished resistor,

Figure 9 shows a preferred construction of terminal cli and Figures 10 and 11 illustrate the application of the method of the invention to the simultaneous production of a number of similar resistors from a single sheet of lass. g Referring now to the drawings and first to Figures 1-7 thereof, 1 (Figure l) is a glass sheet which is to form the base of the resistor, and which is first rendered chemically clean by immersion for a period of about 48 hours in a suitable acid cleaning bath, such as a 20% aqueous chromic acid solution and then washed and dried.

To the cleaned surface of this glass sheet 1 is then applied by a spinning method, i. e. by rotating the sheet in a horizontal plane at high speed around a vertical axis, a solution comprising a mixture of the sulphoresinate of gold and the sulphoresinate of platinum with gold and platinum in the ratio of 80:20 dissolved in a suitable solvent mixture, such as that given in the above example, so as to produce a thin film of said solution 'evenly distributed over the entire surface of the sheet.

The coated glass sheet 1 is then fired at a temperature of about 500 C. to decompose and burn away the organic constituents of the solution and leave a firmly adherent film or layer 2 (Figure 2) of 80:20 gold:platinum alloy on said glass sheet 1.

The metallised sheet 1 is now coated with a layer 3 (Figure 3) of a light sensitised photo-engraving glue, preferably by the aforesaid spinning method, and then allowed to dry. The thus coated sheet is then exposed to light through a photographic negative of the required resistor track design, the unexposed portions of the glue layer are removed by washing the glass sheet in cold water and the sheet is then baked at a temperature of 280-300" C. for 20 minutes to harden the glue image and convert it into a resist. The result of the above operations is shown in Figure 4, in which 4 represents the track design in hardened glue.

essence The next step of the process is toirernove the portions of the alloy film 2 which are not covered by the hardened glue design 4. This is effected by immersing the sheet 11in a solution comprising 2 :parts by weight :of :bromine and 98 parts by weight of .amyl lactate. The :result of this operation is shown vin Figure .5.

Opposite parallel bands .5 (Figure .5) of a silver paste are now applied in any suitable manner to form thickening portions and the sheet is then again fired at .a temperature of about 600 C. to burn away the glue resist and the organic constituents of the silver paste to leave exposed the track design in metal and to :cause the metallic silver to adhere to the glass. The resulting article is represented in Figure :6.

The metal track design produced by the above series of steps is :shown in Figure 7 and it will be noticed that it includes at :one end thereof *a series .of looped sections 6, each connected .to the adjacent thickening strip 5 by a connecting line 7.

The opposite side edges of the sheet 1 are'then ground away as at 8, 8, 9, 9 (Figures 7 and 8), "leaving central projecting wedge-shapedportions 10, 1.9 on to which are clamped and soldered terminal clips '11, 11, carrying lead wires 12, 12.

The wedge-shaped portions 10, 10 are formed in the manner diagrammatically illustrated in Figure '8, in which 13 and 14 are two rotatable shafts on each of which is mounted a pair of impregnated diamondwheels 15, 15 and 16, 16 respectively, the peripheries :of the wheels being suitably bevelled as .at 15a and 16a to produce the required wedge-shape. To .form the wedge: shaped portions .10, 10, the sheet .1 is introduced between the pairs of rotating wheels 15, .15 and 16, 16, the two shafts .13 and 14 being moved bodily towards one another to .the required extent during the grinding operation.

'The terminal clips 11, 11, each have the form shown in Figure 9 consisting of a flat strip 11a of brass having upstanding divergent sides 1112 with outwardly projecting edges 11c and a central extension lid. The clips 11 are fitted to the sheet 1 by laying them on the wedge-shaped portions 10, 10 and applying pressure to force the sides 11b of the clips over and around the edges of the portion 10, which operation is facilitated by the presence of the projecting edges 110. The clips 11 are then soldered to the silver bands 5 and the lead wires .12, 12 are soldered to the extensions 11d of the clips.

The resistance value of the tracked current ;path will usually be found within i10% of the .final value required and in order to bring the resistance value to that required, one or more of the connecting lines 7 are cut as indicated by the arrow (Figure '7 by .means of .a suitable mechanical scribing device, so as to add to the total resistance as required.

The finished resistor is finally given .one :or more coatings of an insulating moisture-proof varnish, .such as a silicone resin varnish, and finally baked .at a temperature of 170 C. for two hours.

Mass production Figures 10 and 11 show the application of our improved method to the mass production of high value.

electrical resistors.

As shown in Figure 10, a plurality of 80:20 gol'dzplatie num alloy ribbon-like currentp'aths or track designs 17, produced in the manner hereinbefore described in connection with the mass production of electrical resistors, are formed on a single circular glass disc 18. Bands :of silver 19 (Figure 11) are provided on the disc by the well-known screen printing process, and the disc 18 is then divided into a plurality of rectangular strips, each containing a number of tracked designs, by cutting, by means of a cutting device incorporating a number of diamond wheels, simultaneously along the dotted =lines 20..

As clearly shown in Figure :11, each silver band 19 includes a central wedge-shaped projecting portion corresponding to one of the wedge-shaped portions Est) -:of the finished resistor.

Each individual strip is now similarly cut to divide it into individual rectangular tracked resistor elements, the non-rectangular portions being-discarded.

The opposite edges of each individual resistor element are then ground away around the outlinezof the bands 19, 19a, terminal clips are :applied and the resistance vahae finally adjusted in the manner previously described.

Electrical resistors prepared in accordance with d mass-production method. described above in detail and having .a tracked current path -.of 0.005". in width, separated by.a 0.003" :gap, and 33" in length will be found to have a resistance ref-120,000 ohmsil0% before final adjustment and a temperature coefiicient of resistance of approximately 0.028. V

:'It will be .readily appreciated that our improved method of manufacture :enables "compact flat resistors to be prepared which provide metallicconduction in high value resistors with a very high degree of electrical stability comparable to that obtained with the customary lower value wire-wound type of resistor hereinbefore referred to. v

Moreover, metallic conduction ensures a degree of freedom from noise which is..unobtainable with himtype resistors containing carbon or palladium oxide.

Furthermore, if a plurality of resistors are prepared simultaneously from a single sheet .by the mass production methodformingafeature of the invention, it will be found possible to prepare up to 10.0 resistors at one time having uniform characteristics and having resistance values within .:.10% of the. desired value, final adjustment of the resistance being .readily efiected in the manner described.

The cost of production. will be found to be 'low owing to the fact that a plurality of resistors are prepared in one operation and an extremely thin rnetallising film, of the order of one millionth of an inch only, is employed.

Owing to the flattened contour of our improved resistors,.it will be appreciated that a large number of such resistors may be stacked in a small space, thereby enabling very high resistance values to be obtained in a limited area. The resistors, moreover, will find .rea'dy application in printed circuits.

Whatwe claim is:

1. A method of making a-high value electrical resistor which comprises coating at least one surface of a substantially fiat sheet of non-conducting heat-resisting material, with/a thin adherent metallic film evenly distributedover the surface by spinning thereon a solution comprisinga mixture of the sulphoresinate of gold and the sulphoresimate of platinum with the gold and platinum in the ratio of GO-:90 parts of gold to 10-40 parts of platinum dissolved 'in a solvent mixture therefor; firing the coated sheet to burn away the organic matter and cause the resultant alloy film to :adhere firmly to said sheet; and tracking said alloy film by a photoetching process to narrow the conducting path and increase the resistance.

A method of making a high value electrical resistor which comprises "the steps of coating a substantially flat sheet of non-conducting, heat-resisting material, with a thin uniform adherent film of an 30:20 goldzplatinum alloy by spinning thereon a solution comprising a mixture of a compound of gold and a compound of platinum in the required proportions dissolved in a solvent therefor and firing the coated glass to burn away the organic matter and cause the resultant alloy film to adhere firmly to the glass;-coating-the resulting metallised glass surface with a'film of a light-sensitised photo-engraving glue; processing sheet of non-conducting,

the film photographically to produce thereon a positive image of a track design representing a narrow resistor current path; hardening said image to convert it into a resist; removing the metal not covered by said resist by etching with a buffered halogen etching bath, thus leaving the track design in metal below the resist; and finally firing the so-trea'ted glass to burn away the resist.

3. A method of making a high value electrical resistor which comprises coating the surface of a substantially flat eat-resisting material, with a uniform thin adherent film of a goldzplatinum alloy by spinning thereon a solution of a compound of gold and a compound of platinum and firing to burn away the organic matter; coating the metallised surface with a film of I a light-sensitised photoengraving glue; processing the film photographica'lly to :produce thereona positive image of a track-design representing a narrow resistor current path; hardening said image to convert it into a resist; removing the alloy metal not covered by said resist by etching with a buffered halogen etching bath, thus leaving the track design in metal below the resist and firing the article to burn away the resist.

4. A method of simultaneously producing a plurality of high-value electrical resistors on a single substantially flat sheet oficon-conducting,.heatmesisting material, which 9 comprises coating said sheet, on at least one surface thereof with an adherent thin uniform film of gold:platinum alioy in the proportions of 60-90 parts of gold to 10- 40 parts of platinum by spinning thereon a solution of a compound of gold and a compound of platinum and firing to burn away the organic matter; tracking said alloy film by a photoetching process to provide a plurality of individual narrow ribbon-like current paths, each including a plurality of parallelly arranged looped sections, applying to said so-treated sheet bands of metal separating said individual track designs, dividing the sheet into elongated strips by cutting substantially along the centre line between each of said adjacent metal bands, then cutting each strip substantially transversely of the length thereof to form a number of individual tracked resistors having two opposed thickened edges forming terminals and finally adjusting the resistance value of each resistor as required by introducing one or more of said looped sections into the current path.

5. A method according to claim 1 in which the metallising solution is composed as follows, namely:

Parts Gold sulphoresinate 280 Platinum sulphoresinate 140 Rhodium sulphoresinate 60 Vanadium resinate 70 Boric acid solution in benzyl alcohol 10 Cyclohexanol 250 Colophony 100 Oil of lavender 20 Oil of rosemary 20 Spirits of turpentine 20 Nitrobenzol 30 giving the following percentage composition:

Per cent Gold 5.60 Platinum 1.40 Rhodium 0.14 Vanadium pentoxide 1.14 Boric oxide 0.07 Solvents and resinous constituents 92.65

6. A method of making a high value electrical resistor which comprises the steps of (a) applying to at least one surface of a substantially flat sheet of glass a solution comprising a mixture of a compound of gold and a compound of platinum, with the gold and platinum in the ratio of 4: 1, dissolved in a solvent mixture therefor including a flux; (b) firing the coated sheet at a temperature of about 500 C. to burn away the organic matter and cause the resultant alloy film to adhere firmly to said sheet; coating the resultant metallised glass surface with a film of a light-sensitised photoengraving glue; processing the film photographically to produce thereon a positive image of a track design representing a narrow resistor current path; (d) baking said sheet at a temperature of 280300 C. to harden the glue image and convert it into a resist; (e) removing the metal not covered by said resist by etching with a buifered halogen etching bath, thus leaving the track design in metal be low the resist; thickening the opposed ends of the track design by applying to said sheet a silver paste; (f) firing the so treated glass at a temperature of about 600 C. to burn away the resist and cause the metallic silver to adhere firmly to said glass and finally (g) adjusting the resistance value by a mechanical scribing operation.

7. A method of mass producing high value electrical resistors which comprises the steps of: (a) providing a circular glass disc of predetermined size; (b) cleaning said disc chemically by immersing said disc in a 20% aqueous chromic acid solution for a period of 48 hours, washing and drying; (c) coating said disc one side thereof with a thin uniform film of gold:platinum alloy by applying to said disc a solution comprising a mixture of the sulphoresinate of gold and the sulphoresinate of platinum, with the gold and platinum in the ratio of 4: 1, dissolved in a solvent mixture therefor; (d) spinning said coated disc in a horizontal plane at a speed of 800 R. P. M. for a period of 5 minutes; (e) firing said coated disc by heating said disc up to a temperature of 420 C. during a period of about 70 minutes, maintaining said disc at said temperature of 420 C. for a period of minutes and then allowing to cool, to burn away the orplacing said disc in a buttered halogen ganic matter in said solution and cause the resultant alloy film to adhere firmly to said glass; (1) coating the metallised surface of said disc with a uniform film of a dichromate-sensitised photoengraving glue; (g) arranging in contact with said glue film a photographic negative of a plurality of individual track designs representing narrow ribbon-like conducting paths; and each of said designs including a number of looped sections; (it) exposing said glue film to light through said negative for a period of from 2 to 10 minutes and developing said glue image by washing in cold water and drying; (i) stoving the so-treated disc at a temperature of from 280300 C. for a period of 20 minutes to harden said image into a resist; (j) removing the metal not covered by said resist by etching bath, composed of 98% amyl lactate and 2% bromine, for a period of about 10 minutes at a temperature of about 30 C., thus leaving the individual track designs in metal below the resist; thickening two opposed sides of each track design by applying thereto by the silk-screen printing method a band of silver paste to one of which each of said looped sections is connected; (k) firing said disc at a temperature of 600 C. to burn away the glue resist and to burn away the organic matter in said paste and leave firmly adherent bands of metallic silver at said thickened sides; (I) cutting said disc to form a plurality of individual tracked resistor elements; (m) soldering terminal clips to each of said silver bands and (n) finally adjusting the resistance value of the tracked current path of each resistor by cutting the connection of one or more of said looped sections to its silver contact band so as to increase the resistance.

8. A method of making a high value electrical resistor which comprises coating a substantially fiat sheet of a non-conducting heat-resisting material on at least one surface thereof with a thin adherent metallic film evenly distributed over said surface by spinning thereon a solution comprising a mixture of an organic compound of gold and an organic compound of platinum dissolved in a solvent mixture therefor and firing the coated sheet to burn away the organic matter and cause the resultant alloy film to adhere firmly to the sheet and subsequently tracking said alloy film by a photoetching process to provide a narrow ribbon-like current path.

9. A method of making a high value electrical resistor which comprises coating a substantially fiat sheet of a non-conducting heat-resisting material on at least one surface thereof with a thin adherent metallic film evenly distributed over said surface by spinning thereon a solution comprising a mixture of an organic compound of gold and an organic compound of platinum dissolved in a solvent mixture therefor and firing the coated sheet to burn away the organic matter and cause the resultant alloy film to adhere firmly to the sheet, subsequently tracking said alloy film by a photoetching process to provide a narrow ribbon-like current path, including a plurality of loops connected in parallel to one terminal of the resistor circuit and severing the connection of one or more or" the loops to said terminal to increase the resistance as required.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Clear Nov. 28, 1933 Kisfaludy Nov. 19, 1935 Gaut et al Sept. 23, 1941 Dorn Feb. 24, 1942 lira May 5, 1942 Guellich Nov. 20, 1945 Guellich May 7, 1946 Christensen Apr. 29, 1947 Kerridge Feb. 10, 1948 Kerridge Sept. 30, 1949 Lokker et al. May 9, 1950 Lazzery Sept. 23, 1952 FOREIGN PATENTS Country Date France Dec. 12, 1936 OTHER REFERENCES British Journal of Photography, December 16, 1949, p.

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1. A METHOD OF MAKING A HIGH VALUE ELECTRICAL RESISTOR WHICH COMPRISES COATING AT LEAST ONE SURFACE OF A SUBSTANTIALLY FLAT SHEET OF NON-CONDUCTING HEAT-RESISTING MATERIAL, WITH A THIN ADHERENT METALLIC FILM EVENLY DISTRIBUTED OVER THE SURFACE BY SPINNING THEREON A SOLUTION COMPRISING A MIXTURE OF THE SULPHORESINATE OF GOLD AND THE SULPHORESINATE OF PLATINUM WITH THE GOLD AND PLATINUM IN THE RATIO OF 60-90 PARTS OF GOLD TO 10-40 PARTS OF PLATINUM DISSOLVED IN A SOLVENT MIXTURE THEREFOR; FIRING THE COATED SHEET TO BURN AWAY THE ORGANIC MATTER AND CAUSE THE RESULTANT ALLOY FILM TO ADHERE FIRMLY TO SAID SHEET; AND TRACKING SAID ALLOY FILM BY A PHOTOETCHING PROCESS TO NARROW THE CONDUCTING PATH AND INCREASE THE RESISTANCE. 